Method and apparatus for fabricating screens for television picture tubes



Nov. 1 1 61 H. HEIL 3,008,390

METHOD AND APPARATUS FOR FABRICATING SCREENS 2 FOR TELEVISION PICTURE TUBES 2 Sheets-Sheet 1 Filed Dec.

INVENTOR HANS HEIL,

HIS ATTORNEY.

Nov. 14, 1961 H. HEIL 3,008,390

METHOD AND APPARATUS FOR FABRICATING SCREENS FOR TELEVISION PICTURE TUBES 2 Sheets-Sheet 2 Filed Dec. 27, 1955 4 P I IIIIIIIIIIIII B B F INVENTOR HANS HEIL, BYW 2 V ms ATTO NEY.

States atent Patented Nov. 14, 1961 3,008,390 METHOD AND APPARATUS FOR FABRICATING SCREENS FOR TELEVISIDN PHCTURE TUBES Hans Heil, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 27, 1955, Ser. No. 555,368 4 Claims. (Cl. 951) This invention relates to color television picture tubes, and more particularly to improved methods and apparatus for properly locating phosphor elements on the screens thereof.

Some color television picture tubes comprise an image reproducing structure including a pair of electrodes adapted for being maintained at different potentials to define an electric field thercbetween, and arranged in adjacent relation to the luminescent screen of the tube and in parallel spaced relation to each other, thereby to define an interelectrode region for determining the trajectory and forces of impact of electrons directed toward the screen for impinging upon and exciting the screen. Generally, the image reproducing structure comprises a plate supporting a screen made up of a plurality of groups of different color-producing luminescent materials and bearing a conductive coating. Electron beams from a suitable source are caused to convenge at an apertured mask comprising as for example, a parallel wire grill or an apertured plate which is placed in the path of the beams and in front of the phosphor screen. The beams are modulated simultaneously with color signals and are deflected by beamdeflection means to sweep the screen. The apertured mask interposed between the electron source and the phospher screen serves as a means to prevent each of the electron beams from striking anywhere other than a particular color-producing phosphor element. Therefore, it is important that the apertured mask be correspondingly aligned with the phosphor elements of the screen. It has been found that the problem of alignment is substantially reduced by providing the aforementioned electric field between the screen and the mask. The field between the apertured mask and the conductive coating then operates as electron lenses which focus the electron beams upon corresponding elements on the surface of the screen and it is thus possible to construct masks of which the grill wires may be made thinner, or of which the apertures can be made larger resulting in increased screen brightness in the ratio of the increase of electron transmission through the mask.

, While the use of said electric fields has reduced the problem of alignment, it has given rise to a new difliculty. The field between the mask and the screen produces a deflection of the electron beams when the direction of the field is transverse to the path of the beam. As a result, the beam, in its path between the mask and the screen, is deflected from a straight line path. The phenomenon becomes more pronounced as the angle between the axis of the tube and the beam path becomes larger, as will be explained in more detail subsequently. Such deflection of the beam may become sufiieient to cause the electrons of the beam associated with one color to impinge upon the phosphor element which normally should be excited by electrons of the beam associated with a different color and results in undesired color impurity.

Accordingly, it is a principal object of my invention to provide a method and apparatus for producing color television picture tubes wherein individual phosphor elements on the screen are so positioned as to compensate for undesired beam displacement effects.

It is another object of my invention to provide an effective, efficient and inexpensive method and apparatus for 2 determining the position of phosphor elements on the screen of a color television picture tube.

Other undesirable defects resulting in color impurity are produced by the non-uniformity of beam deflection caused by the usual yoke imperfections in tubes of this type as Well as by the non-uniform deflections due to dynamic convergence corrections.

It is, accordingly, a further object of my invention to provide a novel method and apparatus for determining the position of phosphor elements in color television picture tube screens to compensate for beam displacement effects including deflections caused by the post acceleration field, non-uniform yoke deflections, and non-uniform deflections due to dynamic convergence corrections.

Further objects and advantages of my invention will become apparent from. the following description and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

In the attainment of the foregoing objects, I provide a novel optical method and lighthouse apparatus including means for determining the positioning of phosphor elements of color television picture tubes to compensate for beam displacement eflects.

For a detailed explanation of my invention reference is made to the accompanying drawings in which:

FIGURE 1 is an isometric view of a tricolor television picture tube embodying the features of my invention;

FIGURE 2 is an enlarged fragmentary isometric view of a portion of the wire grill mask and the phosphor screen of FIGURE 1;

FIGURE 3 is a diagram of electron beam trajectory useful in understanding the invention; 7

FIGURE 4 is a cross section view of an apparatus constructed in accordance with one embodiment of my invention;

FIGURE 5 is a plot of a family of curves to show the effect of the acceleration field;

FIGURE 6 is a diagram useful in understanding the operation of my invention;

FIGURES 7, 7a and 7b are diagrammatic illustrations useful in understanding the embodiment of FIGURE 4;

FIGURE 8 is a cross section view of an apparatus constructed in accordance with another embodiment of my invent on; and

FIGURE 9 is a diagram useful in understanding the em bodiment of FIGURE 8.

In FIGURE 1 there is a tri-color cathode ray image reproducing or picture tube of the post-acceleration type embodying my invention. Tube 11 includes an envelope 12 having a cylindrical neck portion 13. Located in the neck portion 13 there is shown diagrammatically an electron beam producing means 14 which may comprise three electron guns, although it will be understood by those skilled in the art that other electron beam-producing means may be employed. By means not shown, the beams emitted by the electron guns 14 are focused into relatively sharp beams 15. The focused beams are deflected vertically and horizontally by suitable deflecting means 16; the center of deflection being designated as 20. The beams 15 are caused to scan over the opposite or viewing end of the envelope 12. which comprises a screen 17 made up of three different color-producing phosphor materials, i.e., red, green, and blue, and a wire grill mask 18 which will be described in detail subsequently. Screen 17 and mask 18 present substantially parallel surfaces spaced a small distance from each other and normal to the axis of the tube. Each electron gun 14 is adapted for emitting an electron beam intended to impinge and excite a particular color phosphor on screen 17. An electric field between mask 18 and screen 17 which accelerates the electron beam is produced by a power source 19, hence the common name, post-acceleration tube.

Provided for making suitable electrical connections with the electron guns 14 and other electrode elements in the tube are a plurality of conductive pins 21 carried in an insulative base member 22 fitted on the end of the neck portion 13.

\As shown more clearly in FIGURE 2, it will be seen that the surface of the screen 17 is formed of a plurality of triads of vertically aligned phosphor stripes 24, each stripe being adapted to emit a characteristic color when impinged by an electron beam. The triad consists of so-called red, green and blue phosphors depicted as R, G and B, respectively. Since each electron gun emits a beam of electrons representative of one color component of the image to be produced, for color purity to be obtained, it is necessary that only such beams strike those phosphor stripes which produce a color corresponding to the component of color represented by the associated beam. For example, should electron beam 15a be modulated in accordance with signals corresponding to the red component of the picture to be produced and adjusted to impinge upon the R phosphors, beam 15a as it is swept across screen 17 should then always impinge on the R phosphors. As previously stated, in order to prevent each beam from impinging on those stripes which do not correspond in color-producing quality to that represented by that particular beam and to insure that each of the beams impinges only on the proper color stripes, mask 18 comprising a plurality of spaced parallel vertical wires 25, is interposed between the source of the electrons and the screen 1'7. One triad of phosphor stripes on screen 17 is provided to correspond to each vertical aperture between adjacent grill wires 25.

The problem of alignment is essentially the same in color television tubes of the type which use a screen surface comprising a plurality of color elements composed of a triad of dots arranged in substantially equilateral triangles. In the dot type tube the electron guns must be arranged to impinge on particular dots and each beam corresponding to the phosphor dot on the screen must impinge upon that particular dot and not on the other color dots. The apertures in the mask are generally formed such that each aperture in the mask corresponds to one triad of phosphor dots. The invention to be described in detail, it will be appreciated, is applicable to such masks or grills, the aforementioned types being here named merely by way of illustration.

Addressing the problem in detail, it is found that in post acceleration type tubes the field between the grill mask 18 and the screen 17 deflects the electron beams 15 toward the normal to the screen. in FIGURE 3, drawn to an exaggerated scale, for a given beam angle relative to the normal to the screen, a straight line through an aperture in the grill wire mask 18 will establish a point where the electron beam would impinge on the phosphor screen 17 if there were no accelerating field. Because there is an accelerating field between the mask 18 and the screen 17, electron beam 15y, for example, entering the field at a large angle 61 to the normal will be deflected to a greater degree, say 2,, from its original trajectory than will electron beam 15y which enters the field at a lesser angle B2 and is deflected by an amount, say Z less than 2,. vAs will be subsequently discussed, the amount of deflection, Z, is a function of the duration of electron transit time in the acceleration field.

In accordance with my invention, I provide a method and apparatus whereby visible light and relatively simple photographic techniques can be utilized economically to produce master patterns having line or dot location positioned so as to compensate for the beam displacement caused by post acceleration. In FlGURE 4, there is shown in cross section an optical unit which may conveniently be termed a lighthouse and which is adapted for use as a light-tight unit 26 or if desired, can be adapted for use in a dark room. The lighthouse includes a glass plate 27 supporting a light-transmissible ruling or grating 29 of gelatin or the like, and consisting of an array of spaced parallel high and low surfaces 23 and 23', respectively, providing relatively high and low transparency regions. Such high and low surfaces can be obtained by ruling or engraving or otherwise scoring the gelatin material to form the ruling 2.9.

The ruling 29 is interposed between a light sensitive plate 28 having a sensitized lower surface 31, and a conveX-plano lens 32.. A member 34 which has a small aperture therein, and a source of light schematically illustrated by a lamp 33 adapted to radiate through said aperture are disposed so that light from the source 33 traverses the lens 32, the ruling 29, and is incident in the light sensitive surface 31. A cross sectional view of the ruling 29 is here shown greatly exaggerated for purposes of illustration. One practical ruling has a pitch of 32 to the inch and the rulings are .009 inch wide. If desired, ruling 29 be replaced by photographic copies of rulings as is well known in the graphic arts, or, may conceivably be one of the actual masks "18 (FIGURE 1) constructed for use in tube 11.

For cases in which no post-acceleration is employed, the position of the phosphor stripes can be accurately determined merely by adjusting the relative positions of the plate 27 and the sensitive member 31, so that the ratio of d/D is identically the same as that of the corresponding ratio in an actual tube, the quantities d and D being defined as follows:

In the lighthouse unit 26, d is the distance between the plate 27 and the sensitive member 31 and D is the distance between the plate 27 and the effective source of light 34.

In tube 11 of FIGURE 1, d is the distance between mask 18 and screen 17, and D is the distance between the mask 18 and the center of deflection 20 of the beam.

The center of deflection, as is known, is the intersection of the rectilinear extensions of the electron trajectories before and after the deflecting means 16 deflect the electron beams from the initial path established by the electron gums.

The center of deflection is not stationary but varies over a small distance in a manner related to the instantaneous position of the deflected beams, resulting in a non-uniform deflection over the deflecting range of the yoke, thereby effecting the point of impingement of the electron beams on the screen. Since multiple guns are generally used and since the beams of the guns must converge at a point adjacent the mask, it has been found necessary to provide dynamic means (not shown) to converge the eams. The addition of the dynamic convergence means adds a further deflecting effect which must be taken into account when positioning the phosphor elements.

When the light from source 33 is permitted to pass selectively through the openings of the plate 27 and to fall on the light sensitive member 31, a pattern is obtained thereon of the desired phosphor stripe locations. The screen pattern so obtained is similar to the pattern of the openings of the plate 27 and magnified by a factor M with respect to the plate pattern which may be expressed by the formula:

riodic. In the neighborhood of the center, where the light rays impinge on the screen at substantially perpendicular incidence, the screen pattern is similar to the grill pattern but enlarged by a magnification factor different from the one defined at (1) above. However, for beams incident further away from the center of the screen, the pattern becomes denser and varies identically along straight lines going out from the center. As a consequence, the outer phosphor lines must be made slightly curved to compensate for the distortion. It has been found that good color purity considerations require that the phosphor elements be positioned with an accuracy of approximately 0.001 inch over 20 inches of screen. Accordingly, the accuracy provided by this invention makes possible commercial production of color television picture tubes with good color purity.

The magnification factor for the central picture area as extrapolated is determined by the following formula: m= +%'i ;(w +v where M is the magnification factor in a tube utilizing a post acceleration field, and where 'y is the so-called post acceleration factor and is defined as:

where V is the voltage applied to the screen and V is the voltage applied to the mask, each of these voltages being taken relative to cathode potential.

FIGURE 5 is a graph showing a family of curves plotted with ratio r/ d as the axis of ordinates and the ratio R/D as the axis of abscissas, each of the curves shown being derived for individual values of the post acceleration factor 7 by the application of a formula:

Wy ming D which can be derived from a consideration of the initial conditions of an electron entering the space between the plane of mask 18 and the plane of screen 17 at any angle as given by R/ D, and the effect on such an electron by the magnitude of the post-accelerating field as described by the quantity 'y, where, as shown in FIGURE 6, r is the distance measured on the surface of the screen between a line drawn normal to the screen from the point of intersection of the electron beam and the mask, and the point where the beam impinges on the screen, and, R is the distance between the axis of the beam and the point of intersection of the beam with the plane of the mask.

The transit time of the electron in the space between the planes of the mask 13 and the screen 17 can be readily calculated. Since the component of the electron velocity perpendicular to the post acceleration field is unaffected by this field, the distance r is simply the component of travel in this direction (parallel to planes S or M) during the transit time.

The curves of FIGURE 5 are a graphic illustration of the Formula 4 and the curves are substantially linear for small values of R/D and, except for :0, have decreasing slopes for increasing values of the distance ratio R/D. The increased deviation from a straight line characteristic would result in color impurity at the Wider angles unless the phosphor elements are placed in correspondence with the places of impingement of electrons as determined by Formula 4.

The distance ratio R/ D can be simulated in lighthouse unit 26 so that the condition that obtains in an actual tube in which a post accelerating voltage is applied between the mask and the screen may be effectively realized therein. The action of the post acceleration field on the electron beams is, as has been described, to produce a deflection in the trajectory of the beam in the mask-screen region that causes a displacement of the point at which III 6 the beam intersects the screen. In lighthouse unit 26 of FIGURE 4, this condition may be simulated by adjusting the position of the effective source 34 relative to plate 27 and screen 28 to produce an apparent displacement of the point of intersection of the beam and the screen. This adjustment is indicated in FIGURE 4 by the double arrows 39. Of course, the same effect can be obtained by changing the position of the light sensitive member 28 with respect to the source 33 and to plate 27. The altered distance between plate 27 and the eifective source 34 is designated as D for purposes of subsequent detailed explanation. From Equation 4 the value of D can be derived by extrapolating for small values of R and the following formula is found for the value Doom as used in the lighthouse, in relation to the value D, as used in the final tube:

D 2 uorrect To aid the understanding of the invention, FIGURE 6 shows a composite diagram of the paths of a ray of light in the lighthouse of FIGURE 4, and of a beam of electrons in the cathode ray tube of FIGURE 1. In the diagram, the point C represents the center of deflection 20 of the electron beams 15.

A random ray of light from the point C is shown in FIGURE 6 by the dash-dot line L and a beam of electrons from the point C is here shown as a solid line 2. The ray L and beam e are so selected as to intersect in a plane M at a point P. The plane M is here taken as the plane occupied by the mask 18 in FIGURE 1 or the member 27 in FIGURE 4. A second plane is taken as the plane occupied by the screen 17 in FIGURE 1 or the member 28 in FIGURE 4. In the region between planes M and S, dash-dot line L, between points P and P, reppresents the extension of the line L and corresponds to the path of travel of the light in this region. Similarly, curved solid line e in the region between planes M and S, between points P and P, represents the electron beam trajectory in this region. Dotted line e" from point P is a linear extension of line 2. A distance r, is the distance on plane S between the point P and the point of intersection of a line erected from point P and perpendicular to the plane S.

Plane P" is the point of intersection of line e and a line erected from point P perpendicularly to the plane S.

The distance between point P" and plane M is taken as the quantity D defined above. The quantities a, D, and D shown in FIGURE 6 are as defined above. The distance of point P from the axis A-A is here designated as R.

It will be now observed, from an examination of the diagram just described, that, in the lighthouse of FIG- URE 4, if the distance between members 27 and 28 is selected as corresponding to d in FIGURE 6, the distance between the effective source 34 and member 27 must be altered so that the light beam is incident on the member 28 at point corresponding to the point P of FIGURE 6. As so adjusted, the distance between the effective source 34 and the member 27 would correspond to Doormat of FIGURE 6.

It is, of course, possible to make this compensation by properly selecting any combination of these distances either singly or collectively so long as their ratio is in accordance with the formula:

To provide desired compensation for the deflections due to the post accelerating field in the regions characterized by large values of R/D as well as those of small values, a lens 32, as shown, has the center portions of opposite faces substantially parallel to each other and as the distances from the center increase the upper face becomes convex in accordance with numerical calculations relating the amount of compensation desired and the curvature required to produce the same, while the lower face is fiat, as will appear.

The design of the lens 32 may be understood by referring to FIGURES 7, 7a and 7b. The distance between effective light source 34 and lens 32 is designated X. One arbitrarily picked ray of light L is shown by a solid line. The distance between the point where this ray intersects the plane of the lens 32 and the axis AA of the system is designated V. The distance between the point where ray L" intersects plate 27 and axis AA' is designated R. The angle of incidence at 27 is designated a; and angle or also is the angle at the upper portion of the plane of the lens 32. The angle between the axis A-A' and the light ray leaving source 33 is designated [3; angle ,8 also is the angle of incidence at the lower portion of the lens 32. FIGURE 7a shows an enlarged view of an outer section of the lens, the lower and upper surfaces of the lens are designated 36 and 37 respectively. At the points where light ray L" penetrates the surfaces 36 and 37, perpendiculars to the lens surface 36, and 37 are designated 36' and 37' respectively. The angle defined by the surfaces 36 and the tangent to surface 37 is h. Therefore the angle between perpendicular 37 and the light ray is a-)\, whereas the angle of incidence at the lower portion corresponds to angle 5 of FIGURE 7.

The choice of distance X is made such that the lens has practical dimensions. If X is too large, the lens becomes impractically large in diameter, while if X is too small the diameter is small but the amount of curvature and therefore its thickness becomes impractically large. In one practical embodiment X was chosen as 7.5 inches.

After X is selected, a geometrical relation between R and V is established, as will appear. It is seen that distance V is related to R and the angle of incidence a at member 27 as follows:

Now, for each value of R we have calculated the angle or in Formula 4, where tan a is identical to r/d. Also, at this point of the calculation, the above mentioned corrections on the tan a versus R values caused by yoke and dynamic convergence deflections are included.

Having, thus, established a unique co-ordination of a V value to each R value, it is apparent that to each such value we find an angle 5 by which the light ray leaves the light source C. The relation is:

It is advisable for the sake of simplicity of manufacture to grind the lens such that one surface, for instance, the

lower surface 36 is flat and the upper surface 37 is essentially convex.

Next determined is the slope of the upper surface with respect to the lower which was described by the angle 7\ such that the desired total refraction, which changes angle 79 to angle x is obtained. If the index of refraction of the lens material is, n, it can be readily seen from the geometrical optics that the following holds:

tan sin B-sin a x nsin fi-cos or A combination of Equations 8 and 9 gives the slopes as function of the parameter V. Having the slopes of such elemental sections, the curve of the lens 37 can readily be obtained by a numerical integration. From such a curve as a reference, the lens can be properly ground.

In the actual calculations for a practical plano-convex lens, the lens surface was, for initial calculations, considered to be a plane. This idealization is corrected by relocating V values, e.g., the values indicating the distance from the axis AA of the system to the point where the light emerges from the lens. Referring to FIGURE 7b, the lens surface 37 must have the correct slope A where the light ray passes a reference plane 40 which is tangent to the center of the upper surface of the lens and parallel to the plane lower surface 36. Since there is a significant curvature in the upper lens surface 37, the particular slope must actually be ground at a point nearer the center of the lens. That is, the slope at point H is ground at point G on the lens surface so that the light ray at point H which is the point defined by the intersection of reference plane 40 and the perpendicular extension from point H, has the correct slope. The amount of correction I, the distance between point G, the point defined by the intersection of reference plane 4t) and the perpendicular extension from point G, is given by:

J=y tan a (10) where y is the distance from the lens to the reference plane 40, and angle or is the angle of incidence at the reference plane.

When such a lens is used in the lighthouse unit 26 between light source and ruled plate 27, a thickness correction is necessary. However, as it means a relatively small correction of Dcmect and since it is fairly constant for small angles, its variation is neglected and the light source 33 to plate 27 distance is enlarged by a factor.

where n is the index of refraction and t is the center thickness of the glass.

In FIGURE 8 there is shown another embodiment of my invention wherein master patterns may be produced, and in which elements similar to those of FIGURE 4 are given corresponding reference numerals. In this embodiment a flat parallel surface member 38 of glass is employed to provide the necessary compensation. In this embodiment as in the lano-convex lens 32 lighthouse unit of FIGURE 4, any one or all of the members 27 or 31 may be moved or adjusted up or down to deflect the light rays a proper amount to provide correct simulation of the electron beam. For purposes of explanation, reference is made to the diagram of FIGURE 9 in which line a represents the location of the sensitized layer 31 positioned to rest on the upper surface of the flat member 38; line b represents the location of the ruled plate 27; line 1 represents the lower surface of member 38; and line k represents a beam of light from a point light source C". In accordance with this method, one or more plane-parallel pieces of glass or other transparent substances can be interposed between the ruled plate 27 and the sensitized plate 28. For an understanding of this method, assume a single piece of glass 38 is placed adjacent to sensitized layer 31 of member 28, let it further be assumed that the glass has a thickness t and an index of refraction n. The following relation is found:

An inspection of the diagram of FIGURE 9 will show that r is the distance between the point where the ray of light strikes the sensitive layer of the screen and a line drawn normal to the glass surface at a point where the beam enters the glass. Proper selection of the parameters including 11, t and 1 results in a deflection of the ray of light between the lines 1 and line a to correspond to the deflection of the electron beams by the post-accelerating voltage in the mask-screen region in an actual tube.

It should be understood that by selecting a number of layers of glass or other substances, such as for example, air, or water, a greater degree of matching may be obtained. However, the use of a single layer of glass plus a single layer of air was found to produce a suflicient- 1y corrected pattern master for a tube having an angle of sweep of 70 degrees.

It will be understood that although a flat screen is discussed in detail, the phosphor elements can be properly positioned in a curved screen surface by employing my invention and taking into account the known geometrical configuration of the curved screen.

While specific examples have been given in describing details of this invention, it will be understood that they have been given merely by way of illustration and that the invention is not limited thereto.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for producing patterns of screens for cathode ray tubes of the type having a screen of potential V and an apertured mask of potential V between which and the screen an electric field of magnitude acts on electron beams traveling from a center of deflection through said mask to said screen, said apparatus comprising a light source, a target plate spaced from said light source a distance corresponding to the spacing of said screen from said center of deflection and having a light sensitivity such as to form a record of an image when exposed to light in the pattern of the image, a pattern member having a pattern of relatively opaque and relatively transparent areas constituting a replica of said apertured mask and positioned between said light source and said target plate with a spacing from said target plate corresponding to the spacing of said apertured mask from said screen, and a piano-convex lens interposed between said pattern member and said light source for directing light rays from said light source through said pattern member and onto said target plate in a pattern corresponding to the pattern of impingement of said electron beams on said screen, said target plate and pattern member and lens being arranged normal to and on a common axis with said light source and said lens having its plane side facing said light source and having a convex surface curvature such that a tangent to said convex surface at a point on said convex surface at a radial distance V from a common axis makes an angle with the plane surface of the lens,

where sin Bsin a n sin fl-cos oe (2) 17 is the index of refraction of the lens material,

(1) )\=are tan I:

(3) B=are tan (4) X is the axial distance between the light source and lens,

(5) V=Rtan a (D -X) (6) R is the distance between the axis of the tube and a point of intersection of the electron beam with the plane of the mask,

7 tan (i=5 (11) D is the spacing of the mask from the center of deflection of the tube,

( 13) V is the potentlai of the screen, and

(14) V is the potential of the mask.

2. In the fabrication of patterned screens for television tubes of the type having a source of electron beams, an apertured mask between which and the screen is main" tained an electric field for acting on said electron beams, and means for deflecting said electron beams from a center of deflection, the method for determining the proper location of the elements of the screen pattern with rela tion to the apertures in the mask comprising simulating said electron beams by light rays, and optically altering the paths of said light rays to vary the position of the virtual source thereof to correspond to deviation from a straight line of said electron beams between said deflection means and said screen.

3. In the fabrication of screens for cathode ray tubes of the type having a source of electron beams, an apertured mask between which and the screen is maintained an electric field for acting on said electron beams, and means for deflecting said electron beams from a center of deflection through said mask to said screen, the method for determining the proper location of the elements of said screen pattern with relation to said apertures in the mask comprising simulating said electron beams by light rays, passing said light rays through a lens to refract said rays to correspond to electron beam deflection caused by said electric field, passing said light rays through a replica of said apertured mask, and impinging said rays on a photo recording surface.

4. Apparatus for producing patterns 'of screens for cathode ray tubes of the type comprising a source of electron beams, a phosphor screen, an apertured mask between which and the screen is maintained an electric fieid for acting on said electron beams, and means for deflecting said electron beams from a center of deflection, said apparatus comprising a substantial point source of light rays, a target plate spaced from said source and having a light sensitivity such as to form a record of an image when exposed to light in the pattern of the image, a patterned member having a pattern of relatively opaque and relatively transparent areas constituting a replica of said mask and positioned between said light source and said plate, and a lens interposed between said plate and said source, said lens having a plano-convex shape with a single convex aspherical surface facing said patterned member and a planar surface facing said light source and having a focal plane spaced from said target plate, said lens constituting means to retract light rays incident upon said target plate toward the normal to said target plate at each respective point of incidence and direct light rays from said light source through said patterned member and onto said target plate in paths having a virtual source further spaced from said target plate than the actual spacing of said light source from said target plate.

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